In parallel-hybrid systems (for example VW Touareg Hybrid, Porsche Cayenne S Hybrid, Nissan Fuga Hybrid), the internal combustion engine (VM) and the electric machine (EM) are located on one driveshaft (see FIG. 1). In order to be able to drive purely electrically, the internal combustion engine is usually separated from the rest of the drive by a proportional clutch (K0). As a result, only with the aid of the electric machine is it possible to generate propulsion and avoid the drag torques of the internal combustion engine and mass throughput in the exhaust gas post-treatment system.
Clutches can be differentiated on the basis of their design into a pulse and a proportional clutch. Pulse clutches only have the states of open (no torque) and closed (full torque). In contrast, proportional clutches are capable of setting any torque of the clutch characteristic curve between the states of open and closed. They can be used, in particular, in “slip-sensitive” drive trains. Drive trains with a converter are, for example, “slip-sensitive” since the change in the difference in the rotational speed between the electric machine and the transmission input (converter slip) results in an effect on the torque at the wheel.
In order to be able to switch on the internal combustion engine again when necessary during the electric travel without using an additional starting unit (for example pinion starter), the internal combustion engine is started using the clutch. In order to be able to allow for the applicable comfort requirements in a “slip-sensitive” drive train, it is necessary to know as precisely as possible the torque which is set across the clutch since said torque is compensated by means of the electric machine so that the rotational speed of the electric machine and therefore the slip as far as possible does not change in the case of starting.
The prior art comprises carrying out, in a number of starting phases while fulfilling specific peripheral conditions, the balancing of the angular momentum, already described in laid-open patent application DE 10 2008 042 685 A1, of the involved drive train components in order to adapt the clutch torque. In highly simplified terms, during this balancing the expected change in rotational speed at the electric machine is compared with the actual change in rotational speed in the case of starting. If a deviation occurs, the clutch torque of the clutch which is actually transmitted is considered to be a cause of this deviation and is corrected (adapted) in accordance with this deviation.
The adaptation frequency per start is approximately 30% of the starts in the current hybrid series projects (PL72h). On average, approximately four starts occur per kilometer. The more precise the knowledge about the clutch characteristic curve (clutch torque plotted against actuator travel), the more comfortably the restarts can take place. It is therefore necessary to be able to carry out the adaptations as far as possible without restriction and if necessary.